This invention relates to the establishment and distribution of cipher keys in a cryptographic system.
Cryptographic systems are now gaining favor, both for voice as well as data transmission. In such systems it is typically necessary that the parties to a particular transmission each have cryptographic keys to encrypt and decrypt the cipher transmissions. It follows that a compromise to a cryptographic key will in turn reduce the security of subsequent transmissions involving that key. Thus, great precautions must be taken to distribute the cryptographic keys among the system users. Such distribution, for example, using secure couriers to manually update the keys may be possible when the community of users is priorly known but becomes increasingly more difficult when either the number of parties is large or parties who seldom communicate with each other wish to do so. The responsibility for keeping the cryptographic key secure after distribution rests with each user and the longer the key remains effective the greater the risk of it becoming compromised.
Thus, from a practical point of view it is desirable to have the cryptographic key effective for a single session, requiring a new key for each new session. When couriers are used, however, this becomes costly and time consuming, especially when a party wishes to place many secure calls or have many secure sessions.
Attempts have been made to electronically distribute cryptographic keys between users from a key distribution center. One such example is shown in Rosenblum U.S. Pat. No. 4,182,933, issued Jan. 8, 1980. While such attempts have found some degree of success they all suffer from the problem that they are subject to compromise because they usually rely on the security of the transmission media between the key distribution center and the terminal for the distribution of session key information. Thus, an intruder need only compromise the key distribution channel to obtain subsequent session keys. Elaborate systems have sometimes been established to detect such a compromise, all of which are either costly or minimally effective.
Another problem with key distribution centers is that the center can derive the information used to decrypt the secure data exchange between users and thus could theoretically monitor the secure session transmission.